The present invention generally relates to a device and a method for measuring properties of underground water, the device being inserted into an excavation hole, such as a boring hole excavated underground. More particularly, this invention relates to a device and a method for measuring properties of underground water which is capable of precisely measuring water properties over an extended period of time.
In assessing the safety of a structure built deep underground, a survey of underground water flowing around the structure is indispensable. For this reason, a number of boring holes are drilled into the ground around the structure to provide measuring points at which various properties of the underground water can be measured over an extended period of time. Therefore, various conventional means have been proposed and carried out to measure the properties of the underground water. One method to obtain such measurements is to sample underground water in the bored hole from a sampling opening fixed in the bored hole between a pair of packers. The various properties of the sampled water are measured by a sensor placed on the surface of the ground. Alternately, a casing is inserted in the bored hole fixed by a plurality of packers and a sensor is placed in the casing to measure the various properties within the bored hole.
With conventional methods in which the water sampling opening is disposed in the bored hole, the water partioned between an adjacent pair of packers may become mixed with water originating at location above the sampling location which was introduced during installation of the apparatus. Thus, such methods require a prolonged equilibration period over which the water at a given location is continuously sampled until it becomes identical with the water produced at that location. For this reason, sample water obtained for up to three days after installation of the apparatus cannot be used for generation of data. It is therefore necessary to continuously monitor the obtained water samples until it is determined that an equilibrium state has been reached at which the obtained water sample are equivalent to the water that enters the hole at that level. This type of method, however, has the following problems:
1 Even if the hole is partitioned by two packers and the water is sampled on a continuous basis, the water present in the space partitioned by the packers at the time of installation may not be completely displaced. Some of the old water may remain, thus making it uncertain whether the sampled water represents actual underground water from that level, even though the water from that level is subjected to monitoring by a water quality sensor.
2 When a pump is pumping up water at a constant rate, the underground water pressure in the space partitioned by the packers rapidly falls if the pumping rate is faster than the supply rate of water from the surrounding ground, causing the level of O.sub.2 and CO.sub.2 dissolved in the underground water to change their values and thereby distorting the measured values for the water's properties.
3 If the underground water is rich in sulfides, these constituents will deposit, precipitate on, or corrode the electrodes of a pH sensor or an oxidation reduced potential sensor, also resulting in distorted measurements.
4 The determined value of the sampling position is not entirely dependable.
5 Because the device is quite long, it is difficult to smoothly insert the apparatus into a hole if the hole is not straight.
In the conventional measuring device of a method that fixes a casing in the bored hole, it is difficult to use a casing with optional diameters because a specific casing diameter must be used. It is also impossible to use a conventional device capable of continuous sampling because the space inside the casing is small. Moreover, it is impossible to sample the water continuously and in large volume because a sampling bottle must be lowered into the small space and the samples must be taken one bottle at a time. In addition, a sample characteristic such as water temperature may change before it is measured above the ground.